In industrial measuring circuits, signals must be reliably transmitted from sensors, transmitters and field devices to the PLC, controller or control system. In practice, however, this is not always as simple as it looks in the circuit diagram. Potential differences, ground loops, EMC interference, long cables, different grounding points or incorrectly combined signal types can cause a 4–20 mA signal, a 0–10 V signal or a temperature signal to become implausible.
Galvanic isolation is therefore an important topic in control cabinet construction, process automation and troubleshooting in existing systems. It ensures that two circuits are electrically separated from each other while the measurement signal is still transmitted. This can reduce interference, avoid potential carry-over and protect downstream devices.
This article explains when a simple direct connection is sufficient, when an isolation amplifier is useful and when a transmitter is required. It also shows typical fault patterns in 4–20 mA current loops, 0–10 V signals and temperature measurements, as well as practical diagnostic options on site.
Table of contents
- Basics: What does galvanic isolation mean?
- Typical problems: potential differences, ground loops and EMC
- Which measurement signals are particularly affected
- When an isolation amplifier is needed
- When a transmitter is the better solution
- Temperature measurement: Pt100, thermocouple and galvanic isolation
- Planning galvanic isolation correctly in the control cabinet
- Diagnostics: How can a ground loop be identified?
- Practical example: 4–20 mA signal fluctuates despite a stable process
- Which measuring instruments / products are suitable?
- Conclusion: galvanic isolation is often cheaper than lengthy troubleshooting
- FAQ: Frequently asked questions about galvanic isolation of measurement signals
Basics: What does galvanic isolation mean?
Galvanic isolation means that two electrical circuits have no direct conductive connection to each other. The measurement signal is still transmitted, for example via internal electronic coupling, transformer transmission, optocouplers or other insulating methods. For the user, the decisive point is that the input, output and often also the auxiliary power can be electrically isolated from one another.
In measurement technology, this is particularly important because field devices and control systems are often not at the same electrical potential. A sensor in a plant may be connected to a different potential than the control cabinet via pipework, machine frame, grounding or shielding. If these potentials are directly connected via signal cables, equalizing currents can occur. These currents then flow where they do not belong: through measurement cables, shields or ground connections.
The consequences in practice vary greatly. A measured value may have a slight offset, fluctuate strongly, fail sporadically or only become incorrect under certain operating conditions. What makes this especially tricky is that the interference often does not occur permanently. It may depend on pump operation, frequency converters, motor starts, switching operations, humidity, temperature or plant conditions.
A galvanically isolated isolation amplifier or transmitter interrupts this unwanted conductive connection. The measurement signal is passed on as a standardized signal without the interfering potential differences being transferred directly to the input or output side. This makes the measuring circuit more stable and easier to control.
Typical problems: potential differences, ground loops and EMC
In many plants, measurement signal problems are not caused by a defective sensor, but by the electrical environment. A classic cause is potential differences. They occur when different parts of a plant, control cabinets or field devices have different grounding points. A voltage can exist between these points. If this voltage is equalized via the signal cable, it affects the measurement signal.
A ground loop occurs when a signal or shield is grounded or connected to ground at several points. This creates a closed current path. Equalizing currents can flow in this path and superimpose themselves on the actual measurement signal. This can be particularly noticeable with sensitive voltage signals such as 0–10 V or mV signals, but 4–20 mA current loops are also not fundamentally immune to all installation errors.
EMC interference also occurs when cables are routed in parallel with motor cables, frequency converters, contactors, solenoid valves or powerful consumers. Fast switching operations and electromagnetic fields can couple interference voltages into measurement cables. A clean control cabinet layout, suitable shielding, separate cable routing and galvanic isolation all work together here.
| Fault pattern | Typical cause | Why galvanic isolation can help |
|---|---|---|
| Measured value fluctuates without a process change | EMC coupling, frequency converter, unstable ground references | Isolation reduces the transfer of interfering potentials to the signal. |
| Measured value has a constant offset | Potential difference between field device and PLC input | An isolation amplifier electrically decouples input and output. |
| Signal jumps when motors are switched on | Switching spikes, equalizing currents or poor cable routing | Galvanic isolation can stabilize the measuring circuit and limit interference. |
| Several measuring points influence each other | Common ground, unsuitable supply or shared return conductors | Separate signal paths prevent unwanted coupling between measuring circuits. |
| Sensor works directly, but not via a long cable | Cable resistance, interference, shielding or grounding problem | Signal conditioning and isolation improve transmission to the control cabinet. |
Which measurement signals are particularly affected
Various measurement signals are used in industry. Particularly common are 4–20 mA, 0–10 V, frequency/pulse signals, resistance signals from Pt100 or Pt1000 sensors, thermocouple voltages and digital communication signals such as HART, IO-Link, Modbus or other fieldbuses. Each signal reacts differently to interference.
4–20 mA is popular in process automation because current signals are more robust against cable resistance than voltage signals. In addition, a cable break can often be detected more easily by a signal below 4 mA. Nevertheless, 4–20 mA loops can also cause problems, for example due to potential differences, incorrectly supplied auxiliary power, common ground paths, unsuitable input cards or faulty shielding.
0–10 V signals are more dependent on cable resistance, ground reference and interference coupling. They are well suited for short distances in a controlled environment, but can be more susceptible over long cables or between different control cabinet areas. With mV signals from thermocouples, susceptibility to interference is even greater because the actual useful signal is very small.
Temperature signals such as Pt100 or thermocouples are often converted into a robust standard signal using transmitters. This is particularly useful when the cable is long, the signal is to be processed in a PLC or galvanic isolation between the sensor side and the control level is required.
| Signal type | Typical application | Special risks | Suitable measure |
|---|---|---|---|
| 4–20 mA | Pressure, temperature, level, flow, process sensors | Ground loops, incorrect supply, scaling errors, potential differences | Isolation amplifier, galvanically isolated transmitter, testing with loop calibrator |
| 0–10 V | Short signal paths, building automation, mechanical engineering | Interference voltage, common ground reference, voltage drop on cables | Signal isolation, conversion to 4–20 mA, clean ground routing |
| mV / thermocouple | Temperature measurement with thermocouples | Very small signal, EMC, compensating cable, reference junction | Temperature transmitter close to the sensor or galvanically isolated in the control cabinet |
| Pt100 / Pt1000 | Industrial temperature measurement | Cable resistance, incorrect 2-/3-/4-wire evaluation, interference | Suitable temperature transmitter, correct connection type, calibration |
| Pulse / frequency | Flow meters, measuring turbines, speed | Signal level, interference pulses, incorrect input, long cables | Signal converter, pulse shaper, isolation and suitable counter input |
When an isolation amplifier is needed
An isolation amplifier is useful when an existing standard signal is basically correct, but must be electrically decoupled, amplified, converted or transmitted with greater immunity to interference. Typical examples are 4–20 mA or 0–10 V signals from field devices that are to be routed to a PLC, controller, display or control system.
The isolation amplifier does not perform the actual physical measurement. It does not measure pressure, temperature or flow itself. It processes an already existing electrical signal. Its task is to galvanically isolate input and output and, if necessary, to scale the signal or convert it into another signal type. For example, a 0–10 V signal can be converted into a 4–20 mA signal, or a 4–20 mA signal can be galvanically isolated and passed on unchanged.
Isolation amplifiers are used particularly often when several systems must be connected to each other. A field device supplies a signal to a local display and at the same time to a PLC. Or a signal must be transmitted from one control cabinet to another. Without galvanic isolation, unwanted ground references can occur. An isolation amplifier provides a defined interface between the plant areas.
Isolation amplifiers are also very useful in retrofit projects. Old sensors, existing wiring and new PLC technology do not always fit together cleanly from an electrical point of view. Instead of rebuilding the entire measuring point, a suitable isolation amplifier can stabilize the signal, adapt it and bring it to the desired level.
When a transmitter is the better solution
A transmitter is required when a sensor signal first has to be converted into a usable standard signal. It is therefore more than a pure isolation amplifier. It processes raw signals from sensors or transducers and generates, for example, 4–20 mA, 0–10 V, HART or another output signal.
Typical examples include temperature transmitters for Pt100, Pt1000 or thermocouples, transmitters for resistances, potentiometers, mV signals, force sensors, strain gauges or electrical measured variables. The transmitter linearizes, scales, filters and monitors the signal. Depending on the version, it also provides galvanic isolation between sensor, supply and output.
A transmitter is particularly useful when the raw signal is sensitive or does not directly match the PLC. A Pt100 can in principle be connected directly to a suitable temperature input card. However, if the cable is long, there are many sources of interference or several different sensor types need to be processed, a temperature transmitter is often the more robust solution.
When selecting a device, it should be checked whether galvanic isolation is actually present and how it is implemented. In many applications, true 3-port isolation is particularly advantageous: input, output and auxiliary power are then isolated from one another. This is helpful in control cabinet construction when the field side, PLC side and power supply need to be cleanly decoupled from one another.
| Device type | Main task | Typical input signals | Typical output signals |
|---|---|---|---|
| Isolation amplifier | Galvanically isolate, transmit or adapt an existing standard signal | 4–20 mA, 0–10 V, 0–20 mA, mV depending on version | 4–20 mA, 0–10 V, 0–20 mA or another standard signal |
| Transmitter | Convert sensor signal into a standardized output signal | Pt100, Pt1000, thermocouple, resistance, potentiometer, mV, process signals | 4–20 mA, 0–10 V, HART or digital interface |
| Temperature transmitter | Evaluate, linearize and transmit temperature sensor signal | RTD, Pt100, Pt1000, thermocouple | Usually 4–20 mA, partly HART or digital protocols |
| Signal splitter | Distribute one signal to several galvanically isolated outputs | Usually 4–20 mA or 0–10 V | Two or more isolated standard signals |
| Process calibrator / simulator | Test, simulate or diagnose signal and measuring chain | mA, V, mV, RTD, TC depending on device | Simulated or measured test signals for commissioning and service |
Temperature measurement: Pt100, thermocouple and galvanic isolation
Temperature measurements show particularly clearly why transmitters can be useful. A Pt100 does not provide a standardized industrial signal, but a temperature-dependent resistance. A thermocouple generates only a very small voltage in the mV range. Both signal types are sensitive to connection errors, cable influences and interference.
With short cables and a suitable input card, direct sensor connection may be sufficient. In many industrial plants, however, the conditions are more demanding. The cable runs through cable trays with motor cables, the sensor is mounted on a grounded machine, the control cabinet is located in another plant area, or the PLC card is not optimally matched to the sensor. In such cases, a temperature transmitter can make the measurement significantly more stable.
A temperature transmitter converts the sensor signal into a robust standard signal. At the same time, it can detect sensor faults, parameterize measuring ranges, linearize the signal and provide galvanic isolation. Especially with long cable runs, it is often useful to install the transmitter close to the sensor or well protected in the control cabinet.
It is important to distinguish between galvanically isolated and non-isolated temperature transmitters. Not every transmitter automatically provides isolation. If potential differences, EMC problems or grounding issues are relevant, galvanic isolation should be explicitly checked in the specification.
Planning galvanic isolation correctly in the control cabinet
In the control cabinet, galvanic isolation should not be considered only as an emergency solution. Especially in systems with many field signals, several power supplies, long cable runs or different grounding concepts, it makes sense to take signal isolation into account already during planning. This simplifies commissioning, maintenance and later expansions.
A clean layout starts with separating power and signal cables. Measurement cables should not be routed unnecessarily in parallel with motor cables, frequency converter outputs or contactor load circuits. Shields must be connected correctly according to the plant concept. The supply of measuring circuits should be clearly structured so that several return conductors are not unintentionally connected to each other.
Isolation amplifiers and transmitters on the DIN rail create defined interfaces. The field side is decoupled from the PLC or control system input. In many devices, the auxiliary power is also isolated. This allows measuring circuits to be checked, replaced and expanded individually without interference being carried over from one area to the next.
For safety-relevant or standards-regulated systems, planning, installation and testing should always be carried out by qualified specialists. Galvanic isolation does not replace proper grounding, surge protection or EMC-compliant wiring. Rather, it is an important component within a clean overall concept.
Diagnostics: How can a ground loop be identified?
A ground loop is rarely visible at first glance. Often, only the result is seen: an unstable measured value, an offset, sporadic failures or differences between the local display and the PLC value. Diagnosis therefore begins with systematic separation of the possible fault sources.
First, it should be checked whether the process value itself is stable. If pressure, temperature or flow is actually fluctuating, the measurement signal is not the cause. The signal should then be compared directly at the transmitter, in the control cabinet and at the PLC input. If the values differ from one another, the problem is often in the cable, supply, input card, scaling or grounding concept.
For 4–20 mA signals, measuring the loop current is particularly informative. It is possible to check whether the current corresponds to the expected process value and whether it behaves the same at different measuring points. If the transmitter outputs correctly but the PLC displays a different value, the cause is more likely to be on the input, scaling or potential side.
The UPS4E loop calibrator is particularly helpful for such tests. It can measure 4–20 mA signals, output defined current values and test current loops with internal supply. This makes it possible, for example, to simulate a transmitter in order to see whether the PLC and control system scale correctly. Likewise, the actual loop current can be measured in order to distinguish sensor faults from wiring or input card problems.
Practical example: 4–20 mA signal fluctuates despite a stable process
In a production plant, the level of a tank is measured via a pressure transmitter with a 4–20 mA output. The process is calm, and the tank level changes slowly. Nevertheless, the PLC shows jumps in the level at irregular intervals. Locally at the transmitter, the display appears plausible. The fault occurs particularly often when a nearby pump starts.
During the inspection, it becomes clear that the signal cable was routed through a cable tray with motor cables. In addition, the shield is connected at several points, and there is a measurable potential difference between the field device and the control cabinet. The loop current fluctuates only slightly, but the PLC display reacts sensitively because there is also an unfavorable input circuit and scaling.
The measuring point is then reassessed. Cable routing is improved, the shielding is connected cleanly according to the plant concept, and a galvanically isolated isolation amplifier is installed between the field signal and the PLC input. A defined 4–20 mA signal curve is then simulated using a loop calibrator. The PLC now displays the values stably and with correct scaling.
This example shows that a fluctuating measured value does not automatically mean that the sensor is defective. Especially with 4–20 mA signals, it is worth looking at the entire measuring chain: transmitter, supply, cable, shielding, equipotential bonding, isolation amplifier, input card and scaling.
Which measuring instruments / products are suitable?
For signal isolation and signal conditioning in the control cabinet, ICS Schneider Messtechnik offers suitable solutions in the field of signal converters. These devices are typically used as DIN rail modules and, depending on the version, can isolate, convert, amplify, split or condition measurement signals for the control level.
If a standard signal such as 4–20 mA or 0–10 V is already available and the main objective is to avoid potential differences, ground loops or interference, isolation amplifiers are the suitable product group. They create defined galvanic isolation between the field device and PLC, controller or display.
If a sensor signal first needs to be converted into a standardized signal, transmitters are useful. They convert input signals such as temperature, resistance, mV, current, voltage or other process signals into standardized output signals. For temperature measurements, special temperature DIN rail transmitters are also available, which can evaluate Pt, Ni, KTY or thermocouple signals and convert them into process signals.
For commissioning, maintenance and troubleshooting, process calibrators and electrical calibrators are helpful. They enable process signals to be tested and simulated before a measuring point is unnecessarily modified or a sensor is replaced prematurely. Especially for 4–20 mA current loops, the UPS4E loop calibrator is a suitable solution for measuring and simulating mA signals, checking the loop supply and verifying scaling on the PLC or control system.
| Product / area | Typical use | Particularly relevant for |
|---|---|---|
| Signal converters | Signal conditioning, signal isolation and conversion in the control cabinet | Process automation, control cabinet construction, retrofit and troubleshooting |
| Isolation amplifiers | Galvanic isolation of existing standard signals | 4–20 mA, 0–10 V, potential differences, ground loops and EMC problems |
| Transmitters | Conversion of sensor signals into standardized output signals | Temperature, pressure, level, flow, humidity, force and strain |
| Temperature DIN rail transmitters | Evaluation and conversion of temperature signals | Pt100, Pt1000, thermocouple, long cables and interference-prone environments |
| Process calibrators / electrical calibrators | Testing, simulation and diagnostics of process signals | Commissioning, maintenance, troubleshooting and calibration |
| UPS4E loop calibrator | Measuring, sourcing and simulating 4–20 mA current loops | PLC scaling, transmitter testing, troubleshooting and signal diagnostics |
Conclusion: galvanic isolation is often cheaper than lengthy troubleshooting
Galvanic isolation is not a special niche topic in industrial measurement technology, but a practical protection against many typical signal problems. Potential differences, ground loops, EMC interference and unfavorable wiring can falsify measured values even though the sensor and PLC are basically working. Especially with 4–20 mA, 0–10 V, temperature and mV signals, it is therefore worth taking a closer look at the entire measuring chain.
An isolation amplifier is useful when an existing standard signal is to be transmitted stably, galvanically isolated or adapted. A transmitter is the right choice when a sensor signal first has to be converted into a standardized output signal. Temperature transmitters are particularly relevant when Pt100, Pt1000 or thermocouples are used over longer cables or in environments with high interference.
The most important recommendation is: Do not immediately replace the sensor if measured values are implausible. First, potential references, shielding, supply, cable routing, input configuration and scaling should be checked. Galvanic isolation using suitable isolation amplifiers or transmitters can permanently stabilize the measuring point and often saves recurring troubleshooting during operation.
FAQ: Frequently asked questions about galvanic isolation of measurement signals
What does galvanic isolation mean for measurement signals?
Galvanic isolation means that two circuits are not directly electrically connected in a conductive way. The measurement signal is still transmitted. This can reduce potential differences, equalizing currents and ground loops. In measurement technology, this often concerns the isolation between the field device and the PLC or control system input.
Why do ground loops occur in 4–20 mA measuring circuits?
Ground loops occur when a measuring circuit is closed via several ground or grounding points. This allows equalizing currents to flow through signal cables, shields or common return conductors. Even with 4–20 mA signals, this can lead to offset, unstable measured values or sporadic interference, especially when several devices with different potentials are connected.
Is 4–20 mA not inherently immune to interference?
4–20 mA is more robust than many voltage signals because the current value is less dependent on cable resistance. However, robust does not mean completely interference-free. Incorrect supply, potential differences, unsuitable shielding, ground loops, defective input cards or incorrect scaling can also affect a 4–20 mA loop.
When do I need an isolation amplifier?
An isolation amplifier is useful when an existing standard signal needs to be galvanically isolated, adapted or transmitted with greater immunity to interference. Typical cases include potential differences between field and control cabinet, 0–10 V signals over longer cables, 4–20 mA signals with ground loops or transmitting one signal to several systems.
When do I need a transmitter instead of an isolation amplifier?
A transmitter is required when the input signal is not yet a usable standard signal. Examples include Pt100, Pt1000, thermocouples, resistance signals, potentiometers or mV signals. The transmitter converts the sensor signal into a standard signal such as 4–20 mA or 0–10 V and, depending on the version, can also provide galvanic isolation.
What is the difference between 2-port and 3-port isolation?
With simple isolation, input and output are often isolated from each other. With 3-port isolation, input, output and auxiliary power are each isolated from one another. This is particularly helpful when field side, PLC side and supply have different potentials or need to be especially cleanly decoupled from one another.
Does galvanic isolation help against EMC interference?
Galvanic isolation can reduce EMC problems, but it is not the only measure. Clean cable routing, suitable shielding, separate routing of power and signal cables, correct grounding and suitable input circuitry remain important. Isolation primarily prevents interference potentials from being carried directly from one circuit to another.
Why are 0–10 V signals often more sensitive than 4–20 mA?
0–10 V signals require a stable ground reference between transmitter and receiver. Voltage drops, interference coupling or potential differences therefore directly affect the measured signal. 4–20 mA is usually more robust for longer cables, but the supply, inputs and potential references must also be correctly designed there.
Can an isolation amplifier correct an incorrect sensor signal?
An isolation amplifier can isolate, scale or convert an existing signal. However, it cannot correct a mechanically or process-related incorrect sensor value. If the sensor itself measures incorrectly, is installed incorrectly or the process connection is clogged, the cause must be eliminated at the measuring point.
Why does the PLC show a different value than the local display?
Possible causes include incorrect scaling, incorrect input configuration, voltage drop, interference coupling, potential differences or an error in the current loop. With 4–20 mA, the actual loop current should be measured and compared with the expected process value. This helps determine whether the fault is in the sensor, the cable or the PLC evaluation.
How do you correctly test a 4–20 mA current loop?
A 4–20 mA current loop can be tested by measuring the actual loop current and simulating a defined current value. A loop calibrator can be used to check whether the transmitter outputs correctly, whether the PLC scales correctly and whether the loop supply is stable. This allows sensor faults to be separated from wiring or parameterization errors.
When should a temperature transmitter be used?
A temperature transmitter is useful when Pt100, Pt1000 or thermocouple signals need to be transmitted over longer cables, integrated into a PLC or protected against interference. It converts the sensor signal into a robust standard signal and, depending on the version, can detect sensor faults, linearize and galvanically isolate.
Is galvanic isolation necessary even with short cables?
With short cables and a clear grounding concept, a direct connection may be sufficient. Galvanic isolation becomes particularly important when several grounding points are present, the field device and control cabinet have different potentials, EMC interference occurs or several systems are connected to each other.
Can galvanic isolation replace poor wiring?
No. Galvanic isolation can reduce interference and mitigate potential problems, but it does not replace proper wiring. Cable routing, shielding, grounding, supply, surge protection and correct terminal assignment must still be planned and implemented properly.
What should be considered when selecting an isolation amplifier?
Important factors include input signal, output signal, supply, accuracy, response time, isolation voltage, 2- or 3-port isolation, installation width, temperature range and approvals. It must also be checked whether the device should only isolate or also provide signal conversion, splitter function, limit monitoring or diagnostic functions.
How should recurring signal interference be handled?
Recurring signal interference should be investigated systematically. First, the process value, sensor signal, supply and scaling are checked. Then cable, shielding, grounding and equipotential bonding follow. If potential differences or ground loops are likely, galvanic isolation using isolation amplifiers or suitable transmitters should be planned.